How to Wire an Alternator

[a64pilot]

Silly question, you removed this and had it re-built?
How did it work before without a ground? Same Alt going back in?

[Dockhead]

Originally Posted by a64pilot

Silly question, you removed this and had it re-built?
How did it work before without a ground? Same Alt going back in?

It worked like a champ for six years. I guess it produced a jillion kW/hours of power over that time.

My guess is that this cable was on there; I just missed it when I put it back in.

I'm not on the boat right now; in London on business. I'll be back tomorrow and check out this theory.

[Dockhead]

. . . and that was the problem.

I found the ground cable in the bilge, put it on, and presto. 65 amps (equivalent to 130 amps on a 12v system) at just over idle. About 1.5kW.

The "no charge" light and blower still don't come on, so there's still a problem with the wiring. But I will worry about that tomorrow. At least the bloody alternator works. I feel like an idiot

[Stu Jackson]

Don't feel bad, we've ALL been there. Refreshing honesty, I'd say.

Good luck with the rest.

[a64pilot]

At least the thing is undamaged.
Bilge blower normally comes on with the ignition, engine doesn't have to be running, just key in run position?


Sent from my iPad using Cruisers Sailing Forum

[Dockhead]

Originally Posted by a64pilot

At least the thing is undamaged.
Bilge blower normally comes on with the ignition, engine doesn't have to be running, just key in run position?


Sent from my iPad using Cruisers Sailing Forum

Engine room fan comes on when power from the "R" terminal on the alternator activates a relay. So not when the key is on, but when the engine actually runs.

The same trigger activates another relay which powers up the windlass.

I am so glad I finally understand these circuits.


I worked some more on this yesterday. The upshot is that the "no charge" lamp is not indeed wired into the excitation circuit. 24v direct to the IGN terminal is what does it. You actually have a choice with my alternator -- put power on via the "no charge" lamp onto D+ (which is "R") -- the classical scheme -- or just put power directly onto IGN with no lamp. There is a resistor inside to deal with the voltage:

http://www.prestolite.com/literature...ng_Diagram.pdf

My wiring diagram was not wrong after all.

I apparently knocked a terminal loose and disabled all the panel lights while fumbling around in the helm panel; so the lack of a "no charge" lamp is unrelated to the alternator problem. I ran the engine for a while today and observed that the alternator is working fine, pumping out 60 and 70 amps even at a fast idle. While doing this, the case runs at about 40 degrees C, and the spindle and hub at 66C, which seems ok to me.


As to the blower -- the relay is bad. Probably ruined when the alternator went down during the summer (off Terschelling Bank, Holland, North Sea, at the end of a two days and two nights passage ). I ordered a new one. And meanwhile, fulfilled a longstanding dream of mine, and wired up a manual override switch.

The manual override is needed when, for example, I arrive in some port and get hooked up to shore power, and need to use the charger intensely but the engine is still heating up the engine room.

[a64pilot]

I understand why you want to run the engine when you use the windlass, but isn't having a relay that is controlled by the alternator adding another failure point, and makes using the windlass impossible if the alternator is inop?
I'm going to wire mine direct, but wondering if I should wire to the starting bank, or the house bank? Starting bank should be at a slightly higher voltage, but the house of course can absorb a big load without dropping voltage as much.


Sent from my iPad using Cruisers Sailing Forum

[ScottMeilicke]

My windlass is on the house bank, wired with it's own circuit, and I like it that way. The battery bank is much larger than the start bank, I can use the windlass without the engine, using a generator, etc. More flexibility.


Sent from my iPhone using Cruisers Sailing Forum

[OceanSeaSpray]

Good on you for getting to the bottom of it the way you did it.


Your temperatures indicate a very cool alternator, it wasn't working hard. They easily reach over 100degC around the stator at high output, especially small frame ones.

[Dockhead]

Originally Posted by OceanSeaSpray

Good on you for getting to the bottom of it the way you did it.


Your temperatures indicate a very cool alternator, it wasn't working hard. They easily reach over 100degC around the stator at high output, especially small frame ones.

Yes, the point of "hot rated" alternators is that they are designed for operating at full output, and that means among other things that the thermal mass of the alternator (mine weighs 14kg or more than 30 pounds) and the cooling system are designed to get rid of the heat generated.

Running car-type alternators at high loads -- for which they are not designed -- and high temperatures, is a recipe for trouble.

[Dockhead]

Originally Posted by a64pilot

I understand why you want to run the engine when you use the windlass, but isn't having a relay that is controlled by the alternator adding another failure point, and makes using the windlass impossible if the alternator is inop?
I'm going to wire mine direct, but wondering if I should wire to the starting bank, or the house bank? Starting bank should be at a slightly higher voltage, but the house of course can absorb a big load without dropping voltage as much.

Without any doubt, the house bank. Electric windlasses draw a lot of power, and you will kill a start batt, and maybe your windlass (amps go up when volts sag, when you drive something like that with an inadequate battery). You want as much battery capacity behind that load as possible.

My boat was originally delivered with four (!) battery banks -- one engine start, one generator start (both big truck starting batteries), one house (4x "leisure" batts), one "service" bank (4x "leisure batts).

The point of the service bank was to service the heavy loads of 4 electric winches, 10 hp bow thruster, and windlass, separate from the other electrical systems.

Now my boat was designed by smarter people than I (Bill Dixon and his guys), and her electrical system is particularly well thought out, so I think ten times before changing anything. But I did combine the "house" and "service" banks, and with apologies to Dixon, it works better, because there is twice as much battery capacity behind the large loads like the windlass. It works well enough that even the thruster can be used without the boost from the alternator, with good results. That's 420 amp/hours (at 24 volts) of batteries.


The interlock with the alternator has two purposes -- first of all, to ensure that the engine is running so that the alternator output is boosting the power available. This is to help prevent damage from voltage sag. My windlass YMMV) draws up to 150 amps or more (at 24v), so output from the alternator makes a big difference.

The second purposes is safety -- to help prevent accidental activation. The windlass is locked out when the engine is off.

But there is a negative safety aspect -- if you have an emergency and need to get the anchor up NOW, why should you have to start the engine?

On balance, I will leave the interlock in place. After all this, I know how to hot wire it if necessary.

[smac999]

Originally Posted by Dockhead

But there is a negative safety aspect -- if you have an emergency and need to get the anchor up NOW, why should you have to start the engine?

On balance, I will leave the interlock in place. After all this, I know how to hot wire it if necessary.

can you drop the windlass without the engine running? that would be a big saftey issue. or does the relay only stop the up button?

[Dockhead]

Originally Posted by smac999

can you drop the windlass without the engine running? that would be a big saftey issue. or does the relay only stop the up button?

On my boat, the interlock stops the windlass completely, both up and down.

You can drop the anchor mechanically, however, by releasing the clutch.

In a real emergency where I needed to get an anchor down, however, I would throw out the Fortress kedge, which I can do with my hands, and which is more likely to bite on the first try.